Cast iron contains carbon as a major alloying element which is usually present in the form of graphite flakes. Processes have been developed wherein graphite-spheroidizing agents are introduced into molten iron prior to casting causing the graphite flakes in the cast iron to assume a spheroidal, i.e., nodular form. This results in a cast iron of increased strength and ductility referred to in the art as ductile or nodular iron. A number of elements (e.g. magnesium, cerium, calcium) are known in the art to be graphite-spheroidizing agents i.e. nodularizing agents. Methods known to the art for the production of nodular cast iron are described, for example, in U.S. Pat. No. 3,905,809--Malizio et.al.
During the casting and solidification of nodular cast iron there is a tendency for iron carbides or chill to form near the surface of the cast iron especially in thin sections of a casting. The formation of these iron carbides is disadvantageous as they can cause the cast iron to be so hard that machining of the casting is impractical. In many instances, the presence of high levels of iron carbide results in castings either being scrapped or requiring expensive heat treatment before machining.
In order to deter the formation of iron carbide in nodular cast iron, various "inoculants" have been developed. An inoculant is a relatively small addition to molten iron, which has been previously contacted with a graphite-spheroidizing agent; the inoculant addition serves to suppress the formation of disadvantageous iron carbides in the cast nodular iron. Inoculants are also used to reduce the amount of the graphite-spheroidizing agent addition required to be introduced into the molten iron without detrimentally affecting the metallurgical or physical properties of the resulting nodular iron product. Typical inoculants known to the art are calcium and aluminum bearing ferrosilicon alloys. It is also known to the art that the addition of magnesium to these calcium and aluminum bearing ferrosilicon alloys increases the effectiveness of the inoculant. Barium is also known to improve the effect of inoculating alloys. Prior art methods for the use of inoculants in nodular cast iron are described, for example, in U.S. Pat. No. 3,995,973--Robinson and U.S. Pat. No. 3,661,566--Percheron et.al.